This invention relates generally to hammermills and, more particularly, to devices for increasing the number of impacts of hammers upon the feed material to be ground.
Hammermills used for grinding or comminuting materials commonly consist of a large housing having a feed material inlet at the top, a grinding chamber below the feed material inlet, and a ground material outlet below the grinding chamber. The grinding chamber is defined by an apertured screen extending downwardly from one edge of the feed material inlet and curving about to form a partly cylindrical surface before extending back upwardly to the other edge of the inlet. The resulting cross-sectional shape is roughly a teardrop formed by a circular lower portion bounded by two straight tangent lines converging toward the edges of the feed material inlet. The apertured screen provides the wall of the grinding chamber and surrounds a rotor mounted coaxially in the cylindrical portion of the grinding chamber. On the rotor, a number of hammer bars are pivotably mounted to be free to swing when the rotor is rotated.
During rotation, the outboard ends of the hammers pass closely along the surface of the apertured screen, impacting upon the feed materials and, thereby, comminuting the materials until the particles are fine enough to pass through the apertured screen to the particle outlet of the housing of the hammermill.
During grinding of a material in a hammermill, the particles of the material, after the first impact of the hammers, very quickly attain the velocity of the hammers tangentially to the screen surface. This is partly due to the impact and partly due to the fanning action of the rotor on the air in the grinding chamber. Of course, the low angle of contact of the particles with the screen prevents passage of even the properly sized particles through the apertures so that the particles travel along the screen surface at approximately the same velocity as do the hammer tips. This results in a very low number of low-energy impacts and an unsatisfactory production rate.
One attempt to alleviate this condition included a U-shaped channel extending axially along the bottom inner surface of the apertured screen. By interrupting the smooth surface of the screen, this disrupts the flow of air and particles along the screen. It slows and deflects particles into the path of the hammers, thereby intensifying the grinding action of the mill. Particles are thus ground more quickly to a finer particle size so that they pass more easily through the apertures of the screen and increase the rate of production. However, one disadvantage of this device has been a great increase in energy consumption for the hammermill per unit of milled feed material. This is due to some of the particles becoming trapped in eddies within the U-channel so that following particles flow past along the screen surface as though the channel were full. This results in the particles losing energy as they slow down. They are then struck by the hammers, but, since they are travelling only slightly slower than the hammers, the impacts do not cause comminution of the particles to the degree necessary for efficient operation. Reduced flow rate and reduced impact energy decreases material spread over the screen and decreases the speed of particles sifting through the screen. In order to achieve the required degree of grinding, additional energy in the form of longer running time must be supplied to the hammers. Aside from the increased energy consumption, there is a significant increase in wear and tear on the hammermill components, as evidenced by increased frequency of maintenance and repairs for a given production level.
The foregoing illustrates limitations known to exist in present agricultural hammermills. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.